1. Field of the Invention
This invention generally relates to systems and methods that provide high-resolution imaging of closely spaced interfaces. More specifically, this invention relates to an interferometric imaging system that substantially increases depth resolution relative to non-interferometric systems.
2. Description of the Related Art
Imaging via time-of-flight tomography is common in many fields of research. It is used in optical coherence tomography (OCT), which has found widespread applications, in part because of the ability to image with high depth resolution. In OCT, for example, this resolution is achieved by using a low-coherence light source, such as a femtosecond optical pulse. See M. R. Hee, J. A. Izatt, E. A. Swanson, and J. G. Fujimoto, xe2x80x9cFemtosecond transillumination tomography in thick tissues,xe2x80x9d Opt. Lett., vol. 18, pp. 1107-1109, 1993. In this case, the depth resolution is determined solely by the bandwidth of the light source. This is a manifestation of the well-known Rayleigh criterion, which relates the achievable depth resolution to the coherence length Lc, which is inversely proportional to the bandwidth. See Y. Pan, R. Birngruber, J. Rosperich, and R. Engelhardt, xe2x80x9cLow-coherence optical tomography in turbid tissue: theoretical analysis,xe2x80x9d Appl. Opt., vol. 34, pp. 6564-6574, 1995. Using broadband optical pulses of 10 femtosecond duration, it is possible to resolve two reflecting surfaces spaced by only a few microns. See D. Huang, J. Wang, C. P. Lin, C. A. Puliafito, and J. G. Fujimoto, xe2x80x9cMicron-resolution of cornea anterior chamber by optical reflectometry,xe2x80x9d Lasers in Surgery and Medicine, vol. 11, pp. 419-425, 1991. To achieve this extraordinary resolution, an arrangement that provides a synchronized reference pulse for a temporal gate is typically employed.
Terahetz (THz) imaging is a rapidly maturing field. Terahertz systems known as terahertz time-domain spectrometers (THz-TDS) often use laser pulses each lasting only 100 femtoseconds (one tenth of a trillionth of a second) to generate, detect, and measure electromagnetic pulses (xe2x80x9cT-raysxe2x80x9d) that each last for about a picosecond (a trillionth of a second, or 10xe2x88x9212s). T-rays can be transmitted through various objects, using an imaging system of lenses and mirrors to focus the T-rays. As the T-rays pass through the object under test, they are typically distorted. These changes in the T-ray signals can be analyzed to determine properties of the object. Materials can be characterized by measuring the amounts of distortionxe2x80x94from absorption, dispersion and reflectionxe2x80x94of the T-rays passing through to a detector. A digital signal processing unit processes the data and translates it into images that appear on a computer screen. The digital signal processor can be programmed to recognize the characteristic shapes of transmitted waveforms and identify the particular material at the spot illuminated by the T-ray beam. This information can be obtained for every point or xe2x80x9cpixelxe2x80x9d on each object.
Because many compounds change T-rays in characteristic ways (e.g., absorption or dispersion), molecules and chemical compounds (particularly in the gas phase), show strong absorption lines that can serve as xe2x80x9cfingerprintsxe2x80x9d of the molecules. T-ray imaging can distinguish between different chemical compositions inside a material even when the object looks uniform in visible light. Although metals and other materials with high electrical conductivity are completely opaque to terahertz radiation, most plastics are transparent to T-rays, so THz systems can xe2x80x9cseexe2x80x9d inside plastic packaging. Many applications of terahertz imaging have been identified, including package inspection, quality control, and gas sensing. One specific application is the semiconductor industry, where detection of very thin or subtle features in packaged integrated circuits is often desired.
In previous work, single-cycle pulses of terahertz radiation have been used for reflection imaging. Because the imaging is performed with short pulses, a three-dimensional image of the object under study can be obtained using a time-of-flight mode. See D. M. Mittleman, S. Hunsche, L. Boivin, and M. C. Nuss, xe2x80x9cT-ray tomography,xe2x80x9d Opt. Lett., vol. 22, pp. 904-906, 1997. Pulses reflected from spatially separated surfaces in the object arrive at the detector at different times. The time delay between adjacent pulses can be related to the distance between the two reflecting surfaces.
One of the unique aspects of the technique of THz-TDS is that it is based on photoconductive or electro-optic sampling, which permits the direct detection of the THz electric field. See P. R. Smith, D. H. Auston, and M. C. Nuss, xe2x80x9cSubpicosecond photoconducting dipole antennas,xe2x80x9d IEEE J. Quant. Elec., vol. 24, pp. 255-260, 1988 and A. Nahata, D. H. Auston, T. F. Heinz, and C. Wu, xe2x80x9cCoherent detection of freely propagating terahertz radiation by electro-optic sampling,xe2x80x9d Appl. Phys. Lett., vol. 68, pp. 150-153, 1996, both of which are hereby incorporated by reference. As a result, the temporal separation between pulses reflected from two closely separated surfaces can be determined directly from the time-domain waveform, without any need for temporal gating. In the previous work on reflection imaging with T-rays, as in OCT, the depth resolution was determined by the Rayleigh criterion. Two surfaces can only be distinguished if the distance between them is larger than Lc/2. Here, the factor of xc2xd arises from the two transits through the intervening medium of the pulse reflected from the farther surface. A depth resolution of xcx9c100 microns was demonstrated using this simple time-of-flight imaging system. See Mittleman 1997. With single-beam time-of-flight techniques such as this, the only way to improve the depth resolution is to increase the bandwidth of the radiation, thereby decreasing Lc.
Methods of terahertz imaging, terahertz reflection imaging, terahertz near-field imaging, and terahertz gas sensing have all been patented within the last few years. See U.S. Pat. Nos. 5,623,145; 5,710,430; 5,789,750; 5,894,125; 5,939,721; and 6,078,047; each of which is incorporated herein by reference.
Given the interest in terahertz imaging and potential applications thereof, it is desirable to provide improvements that enhance the utility of these systems. In particular, terahertz systems with greatly enhanced depth resolution would prove advantageous for the semiconductor industry.
Accordingly, there is proposed herein a broadband system that provides greatly enhanced depth resolution through the use of phase shift interferometry. In one embodiment, the system comprises a transmitter, a beam splitter, a phase inverter, and a receiver. The transmitter provides a transmitted signal pulse that is split by the beam splitter into a measurement pulse and a reference pulse. The measurement pulse is applied to a sample, and a relative phase shift of approximately xcfx80 radians is introduced between the measurement pulse and the reference pulse by the phase inverter. The measurement and reference pulses are then recombined to form a combined pulse that is detected by the receiver. The phase inverter may be a simple lens that introduces a Gouy phase shift by passing either the measurement or reference pulse through a focal point.
The phase inversion causes destructive interference in the combined pulse. This destructive interference is disrupted by perturbations of the measurement pulse by the sample. In this manner, a background-free measurement is provided. This provides a greatly enhanced sensitivity to small changes in the measurement waveform, regardless of the origin of these changes. This technique thus allows for measurements of time delays, changes in the frequency spectrum, and changes in attenuation. Various applications for this technique are described herein.